1. Field of the Invention
The present invention relates to an imaging apparatus using the nuclear magnetic resonance (hereinafter referred to as "NMR"), and more particularly to an NMR imaging apparatus which can completely remove the influence of the non-uniformity of a static magnetic field and the non-linearity of gradient fields on a reconstructed cross section, to prevent the image quality of the image indicating the reconstructed cross section from being deteriorated.
2. Description of the Prior Art
An imaging apparatus using the NMR (hereinafter simply referred to as an "imaging apparatus") determines the density distribution or relaxation time distribution of nuclear spin in a body to be inspected, in an nondestructive manner by utilizing the NMR phenomenon, and reconstructs the cross section of a measuring target of the to-be-inspected body, on the basis of the above distribution. Of various imaging methods which have hitherto been proposed, the projection-reconstruction method and the Fourier transform method succeed in forming a high-quality image of the cross section. Of the two methods, the projection-reconstruction method will be explained below. In this method, a uniform static magnetic field and a gradient field rotatable about the to-be-inspected body are used. Further, at each rotational position of the gradient field, projection of the NMR signal which is obtained in the presence of the intensity distribution of the resulting magnetic field, is detected. This projection is back-projected on the coordinate system indicating the cross section of the measuring target, to form an image. Accordingly, in the case where the static magnetic field is not uniform, or in the case where the intensity change of the gradient field is not linear, each projection deviates from a correct position, and the image is blurred.
The static magnetic field is generated by a permanent magnet or a resistive magnet or a superconducting magnet. In either case, it is difficult to form a uniform static field in a field of view. Further, even when a carefully designated coil or magnet is used, the static magnetic field and gradient field will be subjected to external disturbances if the imaging apparatus is not installed at an appropriate place. In such case, it is difficult to form a uniform static field or a linear gradient field in a field of view. Accordingly, the above-mentioned degradation of spatial resolution due to such non-uniformity of the magnetic field is a serious problem in an actual NMR imaging apparatus. A U.S. patent application Ser. No. 420,303 filed by the present assignee on Sept. 20, 1982, now U.S. Pat. No. 4,472,683, relates to an imaging apparatus using the NMR.